Top 30 Most Common Javascript Problem Solving Questions You Should Prepare For

Welcome to your essential guide for mastering JavaScript interviews. Landing your dream developer role often requires demonstrating strong problem-solving skills, and JavaScript problem solving questions are a core component of technical assessments. This blog post dives deep into 30 of the most frequently encountered questions, covering fundamental concepts, coding challenges, and practical application. Whether you're a junior developer or looking to step into a more senior role, refreshing your knowledge and practicing these JavaScript problem solving questions will significantly boost your confidence and performance. We've broken down each question to help you understand why it's asked, how to approach it, and provide a concise example answer. Prepare to sharpen your skills and impress your future employer with your command of JavaScript problem solving questions. This resource is designed to be a quick reference, enabling you to review key concepts and coding patterns efficiently before your big day. Let's get started and tackle these crucial JavaScript problem solving questions head-on. Mastering these challenges is key to showcasing your analytical abilities and technical proficiency in JavaScript.

What Are javascript problem solving questions?

JavaScript problem solving questions are challenges posed during technical interviews designed to evaluate a candidate's understanding of core JavaScript concepts, data structures, algorithms, and their ability to apply this knowledge to write efficient and correct code. These questions range from theoretical discussions about language features like closures, prototypes, and the event loop, to practical coding tasks such as array manipulation, string processing, and implementing common algorithms. They assess not just memorization, but also the candidate's logic, debugging skills, and ability to articulate their thought process. Interviewers use these JavaScript problem solving questions to gauge how a candidate thinks through a problem, breaks it down, considers edge cases, and arrives at an optimal solution using JavaScript. Success hinges on a solid foundation in JavaScript fundamentals and consistent practice tackling various types of problems.

Why Do Interviewers Ask javascript problem solving questions?

Interviewers ask JavaScript problem solving questions for several key reasons. Firstly, they serve as a reliable indicator of a candidate's technical proficiency and depth of understanding in the language. Knowing the syntax is one thing; applying it effectively to solve problems is another. Secondly, these questions assess problem-solving methodology. Interviewers want to see how candidates approach an unfamiliar problem, analyze its requirements, explore potential solutions, and write clean, maintainable code. It reveals their logical thinking and analytical skills. Thirdly, they help evaluate how a candidate handles pressure and unexpected challenges – crucial skills in a fast-paced development environment. Discussing solutions aloud also demonstrates communication skills and the ability to explain complex ideas clearly. Ultimately, mastering JavaScript problem solving questions is about proving you have the foundational skills and adaptable mindset required to be a valuable contributor to a development team.

Preview List

1. What are the possible ways to create objects in JavaScript?

2. What is the prototype chain?

3. Difference between call(), apply(), and bind()?

4. What is JSON and its common operations?

5. Difference between slice() and splice() methods?

6. How to compare Object and Map?

7. Difference between == and === operators?

8. Write a JavaScript function that calculates the sum of two numbers

9. Write a function to count occurrences of each character in a string

10. Write a function to sort an array of numbers in ascending order

11. Find largest and smallest in an array

12. Write a function to check if a string is a palindrome

13. Write a function to check if a number is prime

14. What are arrow functions and differences from regular functions?

15. What is a closure? Give an example

16. Explain first-class, first-order, and higher-order functions

17. What is currying in JavaScript?

18. What is a pure function? Benefits?

19. Difference between var, let and const?

20. Explain event loop and asynchronous behavior in JavaScript

21. Write a function to remove duplicates from an array

22. How to deep clone an object in JavaScript?

23. Explain difference between null and undefined

24. What is hoisting?

25. Write a function to flatten a nested array

26. How can you implement debouncing in JavaScript?

27. Explain promises and difference between then and async/await

28. Write a function to merge two sorted arrays

29. How to check if an object is empty?

30. What are template literals and how do you use them?

1. What are the possible ways to create objects in JavaScript?

Why you might get asked this:

To assess understanding of fundamental object creation patterns and their underlying mechanics like prototypes.

How to answer:

List the common methods: literals, constructors, Object.create(), and classes. Briefly explain each.

Example answer:

Objects can be made via literals ({}), constructor functions (new Function()), Object.create(prototype), or ES6 classes (new Class()). Each method impacts prototypal inheritance differently.

2. What is the prototype chain?

Why you might get asked this:

Tests knowledge of JavaScript's inheritance model, crucial for understanding object behavior and optimizing code.

How to answer:

Explain that objects link to prototypes, forming a chain traversed for properties/methods, ending at null.

Example answer:

It's how JS objects inherit features. An object links to its prototype, which links to its prototype, etc. JS looks up the chain until a property is found or reaches null.

3. Difference between call(), apply(), and bind()?

Why you might get asked this:

Evaluates understanding of function context (this) manipulation, vital for functional programming and class methods.

How to answer:

Define each: call runs immediately with listed args, apply runs immediately with args in an array, bind returns a new function with this bound.

Example answer:

call(thisArg, arg1, ...) executes function immediately, args listed. apply(thisArg, [argsArray]) executes immediately, args in array. bind(thisArg, arg1, ...) returns a new function with this bound.

4. What is JSON and its common operations?

Why you might get asked this:

Checks familiarity with standard data interchange format used widely in web development (APIs, config files).

How to answer:

Define JSON and explain JSON.stringify() (object to string) and JSON.parse() (string to object).

Example answer:

JSON is a lightweight format for data exchange. JSON.stringify(obj) converts an object to a JSON string. JSON.parse(jsonStr) converts a JSON string back to a JS object.

5. Difference between slice() and splice() methods?

Why you might get asked this:

Assess understanding of common array manipulation methods and their effect on the original array (mutating vs. non-mutating).

How to answer:

Explain slice creates a new shallow copy without changing the original, while splice modifies the original array.

Example answer:

slice(start, end) returns a new shallow copy of a part of an array without changing the original. splice(start, deleteCount, items...) modifies the original array by removing/adding elements.

6. How to compare Object and Map?

Why you might get asked this:

Evaluates knowledge of newer data structures and their advantages over traditional objects for key-value pairs.

How to answer:

Highlight key differences: key types (any vs. string/symbol), iteration order, and built-in size property.

Example answer:

Maps allow any data type as keys, Objects only strings/Symbols. Maps preserve insertion order, Objects don't guarantee it. Maps have a .size property, Objects don't.

7. Difference between == and === operators?

Why you might get asked this:

A fundamental question testing understanding of type coercion and strict comparison, crucial for preventing bugs.

How to answer:

Explain that == performs type coercion, potentially leading to unexpected results, while === requires both value and type to match strictly.

Example answer:

== performs loose equality comparison with type coercion. === performs strict equality comparison; both value and type must match without coercion. Use === to avoid unexpected behavior.

8. Write a JavaScript function that calculates the sum of two numbers

Why you might get asked this:

A basic check on function syntax and simple arithmetic operations. Often an easy warm-up problem.

How to answer:

Write a simple function that takes two arguments and returns their sum.

Example answer:

function sum(a, b) {
  return a + b;
}
// Example usage: sum(2, 3) would return 5

This function takes two numbers and returns their addition.

9. Write a function to count occurrences of each character in a string

Why you might get asked this:

Tests ability to iterate strings, use objects for mapping/counting, and handle initial values.

How to answer:

Iterate through the string, using an object to store counts for each character.

Example answer:

function countCharacterOccurrences(str) {
  const charCount = {};
  for (let char of str) {
    charCount[char] = (charCount[char] || 0) + 1;
  }
  return charCount;
}
// Example: countCharacterOccurrences("hello") returns { h: 1, e: 1, l: 2, o: 1 }

10. Write a function to sort an array of numbers in ascending order

Why you might get asked this:

Checks understanding of built-in array methods, specifically sort, and how to use comparison functions.

How to answer:

Use the built-in sort method with a comparator function (a, b) => a - b.

Example answer:

function sortNumbersAscending(arr) {
  return arr.sort((a, b) => a - b);
}
// Example: sortNumbersAscending([3, 1, 4, 1, 5]) returns [1, 1, 3, 4, 5]

The sort method modifies the array in place.

11. Find largest and smallest in an array

Why you might get asked this:

Evaluates basic array traversal and comparison logic. Can be solved iteratively or using Math.min/Math.max with spread syntax.

How to answer:

Iterate through the array, keeping track of the maximum and minimum found so far.

Example answer:

function findLargestAndSmallest(arr) {
  if (arr.length === 0) return {};
  let largest = arr[0], smallest = arr[0];
  for (let i = 1; i < arr.length; i++) {
    if (arr[i] > largest) largest = arr[i];
    if (arr[i] < smallest) smallest = arr[i];
  }
  return { largest, smallest };
}

12. Write a function to check if a string is a palindrome

Why you might get asked this:

A common string manipulation problem testing reversal and comparison logic.

How to answer:

Reverse the string and compare it to the original. Explain edge cases like empty strings.

Example answer:

function isPalindrome(str) {
  const cleanStr = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleanStr === cleanStr.split('').reverse().join('');
}
// Example: isPalindrome("racecar") returns true. Handles case/non-alphanumeric.

13. Write a function to check if a number is prime

Why you might get asked this:

Tests basic number theory logic and optimization (checking divisibility up to the square root).

How to answer:

Handle base cases (<= 1), then iterate from 2 up to the square root of the number, checking for divisibility.

Example answer:

function isPrime(num) {
  if (num <= 1) return false;
  for (let i = 2; i <= Math.sqrt(num); i++) {
    if (num % i === 0) return false;
  }
  return true;
}
// Example: isPrime(7) returns true, isPrime(10) returns false.

14. What are arrow functions and differences from regular functions?

Why you might get asked this:

Assesses understanding of modern JavaScript syntax and its implications for this binding and arguments.

How to answer:

Describe arrow function syntax and highlight key differences: no own this, no arguments object, cannot be used as constructors.

Example answer:

Arrow functions (=>) provide concise syntax. Unlike function declarations, they don't have their own this or arguments binding; they inherit them from the surrounding scope. They can't be used with new.

15. What is a closure? Give an example

Why you might get asked this:

A fundamental concept in JavaScript, testing understanding of scope and function persistence.

How to answer:

Explain that a closure allows an inner function to access variables from its outer function's scope even after the outer function has finished executing. Provide a simple counter example.

Example answer:

function createCounter() {
  let count = 0;
  return function() {
    count++;
    return count;
  };
}
const counter = createCounter(); counter(); // 1

A closure is when a function "remembers" and accesses variables from its outer scope, even after the outer function has returned.

16. Explain first-class, first-order, and higher-order functions

Why you might get asked this:

Tests understanding of functional programming concepts in JavaScript.

How to answer:

Define each term: first-class (treated as values), first-order (don't take/return functions), higher-order (do take/return functions).

Example answer:

First-class: Functions are treated like values (assigned to vars, passed as args). First-order: Neither take nor return functions. Higher-order: Take functions as arguments or return functions.

17. What is currying in JavaScript?

Why you might get asked this:

Evaluates knowledge of functional programming techniques for transforming functions.

How to answer:

Explain currying as transforming a function taking multiple arguments into a sequence of functions, each taking one argument.

Example answer:

Currying is a transformation that converts a function f(a, b, c) into f(a)(b)(c). It turns a function accepting multiple arguments into a chain of functions, each taking one argument.

18. What is a pure function? Benefits?

Why you might get asked this:

Checks understanding of a core concept in functional programming related to predictability and testability.

How to answer:

Define a pure function (same output for same input, no side effects) and list benefits (predictable, testable, composable).

Example answer:

A pure function always returns the same output for the same input and has no side effects (doesn't modify state or perform I/O). Benefits: easier to test, debug, and reason about.

19. Difference between var, let and const?

Why you might get asked this:

A very common question assessing understanding of variable declaration keywords and their scope and mutability.

How to answer:

Explain scope (var function-scoped, let/const block-scoped), hoisting, and mutability (const prevents re-assignment for primitives).

Example answer:

var is function-scoped and hoisted (declaration). let is block-scoped, not hoisted temporally. const is block-scoped, not hoisted temporally, and prevents re-assignment (though object properties can change).

20. Explain event loop and asynchronous behavior in JavaScript

Why you might get asked this:

Fundamental to understanding how JavaScript handles non-blocking operations in a single-threaded environment.

How to answer:

Describe the call stack, event queue, and event loop's role in processing tasks (like callbacks from async ops) when the stack is empty.

Example answer:

JS is single-threaded. The event loop manages async ops using a call stack and a task queue. It processes stack calls, then checks the queue for callbacks when the stack is empty, enabling non-blocking behavior.

21. Write a function to remove duplicates from an array

Why you might get asked this:

A practical array manipulation task testing common techniques like using Set or iteration with auxiliary data structures.

How to answer:

The most concise way is often using a Set. Alternatively, explain iteration with a frequency map or indexOf.

Example answer:

function removeDuplicates(arr) {
  return [...new Set(arr)]; // Using Set is concise
  // Or: return arr.filter((item, index) => arr.indexOf(item) === index);
}
// Example: removeDuplicates([1, 2, 2, 3, 1]) returns [1, 2, 3]

22. How to deep clone an object in JavaScript?

Why you might get asked this:

Assesses understanding of object copying nuances beyond shallow copies and handling nested structures.

How to answer:

Explain the limitation of shallow copies. Mention JSON.parse(JSON.stringify(obj)) for simple cases and the need for recursion or structured cloning for complex objects.

Example answer:

Simple deep cloning (for JSON-safe objects): JSON.parse(JSON.stringify(obj)). For objects with dates, functions, etc., use a recursive function or the Structured Clone Algorithm (like structuredClone() or libraries).

23. Explain difference between null and undefined

Why you might get asked this:

A basic but essential concept testing understanding of value types and their typical usage.

How to answer:

Define undefined (variable declared, no value) and null (intentional absence of value, assigned explicitly).

Example answer:

undefined means a variable has been declared but not assigned a value. null is an assignment value, explicitly indicating the absence of any object value. It's a value representing "no value".

24. What is hoisting?

Why you might get asked this:

Tests understanding of JavaScript's compilation phase and variable/function availability before execution.

How to answer:

Explain that declarations (var, function) are moved to the top of their scope during compilation, making them available before their line of appearance.

Example answer:

Hoisting is JavaScript's behavior of moving declarations (like var variables or function declarations) to the top of their containing scope during the compilation phase, before code execution.

25. Write a function to flatten a nested array

Why you might get asked this:

A common recursion problem testing array manipulation and understanding of nested structures.

How to answer:

Use recursion or iterative methods (reduce, spread operator with concat) to process nested arrays. Array.prototype.flat() is the modern approach.

Example answer:

function flatten(arr) {
  return arr.reduce((flat, item) => {
    return flat.concat(Array.isArray(item) ? flatten(item) : item);
  }, []);
}
// Example: flatten([1, [2, [3]], 4]) returns [1, 2, 3, 4]. Or use arr.flat(Infinity) for modern JS.

26. How can you implement debouncing in JavaScript?

Why you might get asked this:

Tests understanding of optimizing frequent event handlers (like scroll or resize) to improve performance.

How to answer:

Explain the concept (delaying execution until a pause in events) and provide code using setTimeout and clearTimeout.

Example answer:

function debounce(fn, delay) {
  let timerId;
  return function(...args) {
    clearTimeout(timerId);
    timerId = setTimeout(() => { fn.apply(this, args); }, delay);
  };
}

Debouncing ensures a function isn't called too often. It delays execution until after a set period of inactivity.

27. Explain promises and difference between then and async/await

Why you might get asked this:

Evaluates understanding of modern asynchronous programming patterns in JavaScript.

How to answer:

Define promises (represent eventual result of async op). Explain .then chains sequential actions. Explain async/await as syntactic sugar making promise-based code look synchronous.

Example answer:

Promises manage async results. .then() handles promise resolution/rejection. async/await builds on promises, providing syntax (await pauses execution) that makes async code easier to read, resembling synchronous code flow.

28. Write a function to merge two sorted arrays

Why you might get asked this:

A classic algorithm problem testing ability to use pointers/indices for efficient merging without extra sorting.

How to answer:

Use two pointers, one for each array, iterating and pushing the smaller element to a result array until one is exhausted, then concat the rest.

Example answer:

function mergeSortedArrays(arr1, arr2) {
  const merged = [];
  let i = 0, j = 0;
  while(i < arr1.length && j < arr2.length) {
    if(arr1[i] < arr2[j]) merged.push(arr1[i++]);
    else merged.push(arr2[j++]);
  }
  return merged.concat(arr1.slice(i)).concat(arr2.slice(j));
}

29. How to check if an object is empty?

Why you might get asked this:

A common practical task testing familiarity with object iteration and property checking.

How to answer:

The simplest robust way is checking the number of own enumerable properties using Object.keys() or Object.entries().

Example answer:

function isEmpty(obj) {
  return Object.keys(obj).length === 0;
  // Or: return Object.entries(obj).length === 0;
  // Or: for (const key in obj) return false; return true; (includes inherited, check hasOwnProperty)
}

30. What are template literals and how do you use them?

Why you might get asked this:

Tests knowledge of modern string syntax features for easier interpolation and multi-line strings.

How to answer:

Describe the syntax (backticks) and features (interpolation with ${}, multi-line support).

Example answer:

Template literals use backticks (` `). They allow embedded expressions using ${expression} for string interpolation and can span multiple lines without special characters. Example: const name='Bob'; console.log(\Hello, ${name}!\`);

Other Tips to Prepare for a javascript problem solving questions

Beyond reviewing common JavaScript problem solving questions, several strategies can enhance your preparation. Practice coding regularly on platforms like LeetCode, HackerRank, or Codewars, focusing on JavaScript problem solving questions covering algorithms and data structures. As the legendary computer scientist Edsger Dijkstra said, "The most effective programmer is one who can think about a problem abstractly." Develop the habit of thinking through the problem constraints, potential edge cases, and different approaches before writing code. Always discuss your thought process aloud during mock interviews or practice sessions, explaining your reasoning and alternatives. Consider using tools like Verve AI Interview Copilot (https://vervecopilot.com), which can simulate interview scenarios and provide instant feedback on your communication and technical responses to JavaScript problem solving questions, helping you refine your approach. Another quote to live by comes from Bill Gates: "The computer is the most versatile tool we have. It can be used as a typewriter, a calculator, a library, a mail system, or a creative tool... The possibilities are endless." Embrace this versatility by experimenting with different JavaScript features to solve problems creatively. Leveraging resources like Verve AI Interview Copilot helps simulate the pressure and format of a real interview, allowing you to become more comfortable articulating solutions to complex JavaScript problem solving questions. Practice explaining code, time complexity, and alternative solutions clearly. Consistent practice, coupled with targeted feedback from tools like Verve AI Interview Copilot, is key to mastering JavaScript problem solving questions.

Frequently Asked Questions

Q1: How long should I spend practicing each question?
A1: Focus on understanding the core concept and multiple approaches rather than just memorizing answers.

Q2: Should I memorize code examples?
A2: Understand the logic; don't just copy-paste. Be ready to write code from scratch and explain it.

Q3: How important are time and space complexity?
A3: Very important. Be prepared to analyze your solution's efficiency and discuss trade-offs.

Q4: What if I don't know the answer to a question?
A4: It's okay! Explain your thought process, ask clarifying questions, and try to work through it step-by-step.

Q5: Are behavioral questions also important?
A5: Yes, technical skill is key, but companies also look for communication, teamwork, and cultural fit.

Q6: How can Verve AI Interview Copilot help?
A6: It provides realistic practice and instant feedback on your verbal and coding responses to improve your interview skills.